Project Summary Opioid drugs, such as morphine and oxycodone, are generally the most effective compounds available for treatment of moderate to severe pain, which affects nearly 50 million adults in the United States. In spite of their analgesic efficacy, the use of opioids is limited by an often lethal cluster of side effects: Rapid tolerance leads to quickly escalated dosing, while intolerable withdrawal symptoms build physical dependence. Meanwhile, dose-dependent euphoria encourages abuse and psychological addiction despite an endgame of lethal opioid-induced respiratory arrest. The frequency of clinical pain, coupled with a lack of alternative therapeutic options, has led to a national health crisis centered on prescription opioids. The CDC now reports that a lethal opioid overdose occurs every 15 minutes in the U.S. and a new overdose patient enters a hospital emergency department every minute. In response, state and federal regulatory bodies have recently reduced access to prescription opioids. However, this has also had the effect of forcing many legitimate pain patients toward less effective treatment options in lieu of opioids, often increasing their pain levels substantially. There is, therefore, a clear and pressing unmet need for safer, effective analgesics. To address this need, we are advancing a novel strategy to develop non-addictive analgesics targeting opioid receptor heteromers. We will first develop a diverse set of chemical scaffolds that activate mu-kappa opioid receptor heteromers ? a target shown to produce potent antinociception (?pain? relief in rodents) without physical dependence or drug- seeking behavior. We will do this by accessing two world-class high-throughput screening cores at Boston Children's Hospital and Harvard Medical School and their combined chemical libraries (>500K compounds). We will screen >100,000 of these small molecules to identify activators of mu-kappa receptor heteromers, stably expressed in cells. Next, we will determine the selectivity of these ligands for this target by generating concentration-response curves in cells stably expressing various homomeric (mu, kappa and delta individually) and heteromeric opioid receptors (mu-kappa, mu-delta and kappa-delta). The end goal of this SBIR Phase I award is to identify novel scaffolds suitable for subsequent SAR evaluation and potential advancement toward clinical development.